Free-machining aluminum alloys are well known in the art. These alloys typically include free-machining phases formed from elements such as lead, tin and bismuth for improved machinability. These elements form low melting point constituents which readily melt or are rendered weak due to the frictional heat created during machining. Thus, chip formation during material removal required for the manufacture of complex parts and components is easily facilitated.
These types of alloys generate small chips during the machining process which are easily collected and have minimal adverse impact on the machining process. It is essential that these free-machining aluminum alloys form these small chips for proper machining. Formation of long continuous strips or ribbons is totally unacceptable in machining since the ribbons or strips may wrap around the work piece or machining tool and disrupt the operation. Poor machinability also affects other machining operations since the operator must attend to a single machining operation and cannot effectively supervise numerous operations as is commonly done in practice. AA6061 alloys are generally not optimum for machining since they form these long continuous ribbons during machining.
U.S. Pat. Nos. 2,026,457 and 2,026,575 to Kempf et al. disclose free cutting aluminum alloys. Similarly, U.S. Pat. No. 4,005,243 to Baba et al. discloses a freely machinable aluminum alloy.
Other known machineable alloys include AA6262, AA2011, AA2012 and AA2111.
While the prior art aluminum alloys provide adequate free-machinability, they are not without drawbacks and/or disadvantages. For example, AA6262 contains lead and chips from machining these alloys represent a hazardous waste disposal problem. Casting and production of these alloys presents similar problems.
Prior art alloys containing bismuth, e.g., AA2011 or AA2111, can adversely effect the final mechanical properties of the machined part. Since bismuth has an affinity for magnesium, the bismuth in the alloy has a tendency to combine with the magnesium and prevent or reduce Mg.sub.2 Si formation, which has the potential for reducing precipitation strengthening in AA6000-series alloys.
As such, a need has developed to provide a more environmentally friendly free-machining alloy as well as an alloy that does not have its final mechanical properties compromised by free-machining constituents therein. In response to this need, a free-machining aluminum alloy has been developed which contains indium and tin. The invention further provides a process for making such an alloy.